Found it (I think/hope) — thanks for the mod list, that's what cracked it. It's not a conflict with any single mod, so you can stop bisecting. It was two spots in Beltlayer's load code that scale badly with very large packs: one loop that edited the game's item list while walking it, and a proxy generator that re-scanned every prototype in the game for each entity it touched. Both are fixed in 2.9.45 (and 2.8.31 for the 2.0 branch). Update to that and it should load — let me know if it still hangs and I'll dig further.
